My audio seemed quieter than normal when i upgraded to Win 7 from Vista, and my music even sometimes seemed to change phase, as if someone was moving my speakers towards then away from me, and from left to right. It was strange, but I found that by disabling all enhancments on the sound card options made it louder and back to normal. I had no boxes ticked, but my clicked 'Disable All...'anyway, it cured my problem.
Here are some steps:
1) Right click speaker icon in bottom left (or Control Panel, Hardware & Sound, Sound)
2) Select 'Playback Devices
3) Right click on the speaker output you are using, usually 'Speakers'
4) Select 'Properties'
5) Click 'Enhancements'
6) Tick 'Disable All Enhancements'
7) Click 'OK'
8) Restart Media Player and any other audio programs open, a system restart should not be require, but should done if sound is still not correct.
Hope this helps you and anyone else who is having issues